GENERAL CONSIDERATIONST HE avascularity and transparency of the cornea permit study of early pathologic alterations to a degree not possible in other tissues. The slightest edema or most minute infiltration causes immediate changes in transparency. Alteration in the colloidal properties and the water content of the "corneal gel" leads to increased dispersion and refraction of light and augmentation of the normal Tyndall phenomenon, e. g., increases in relucency. It must be remembered that what is observed grossly or with the slit lamp and corneal microscope is the alteration in optical phenomena which result from changes in the tissues.According to Fischer,1 the normally clear cornea contains 76 per cent of water, and an additional 10 per cent may lead to clouding. When the fluid is gathered in droplets the light is irregularly dispersed, producing a relucent haze. As it affects the epithelium, this is commonly called "bedewing." This